js regex replace match in certain paragraph - regex

Question

---
title: test
date: 2018/10/17
description: some thing
---
I want to replace what's behind date if it's between ---, in this case 2018/10/17. How to do that with regex in JS?
So far I've tried;
/(?<=---\n)[\s\S]*date.+(?=\n)/
but it only works when date is the first line after ---

Answer 1

It is possible though imo not advisable:
(^---)((?:(?!^---)[\s\S])+?^date:\s*)(.+)((?:(?!^---)[\s\S])+?)(^---)
This needs to be replaced by $1$2substitution$4$5, see a demo on regex101.com.
Broken down this reads
(^---) # capture --- -> group 1
(
(?:(?!^---)[\s\S])+? # capture anything not --- up to date:
^date:\s*
)
(.+) # capture anything after date
(
(?:(?!^---)[\s\S])+?) # same pattern as above
(^---) # capture the "closing block"
Please consider using the afore-mentioned two-step approach as this regex is not going to be readable in a couple of weeks (and the JS engine does not support a verbose mode).

Answer 2

Without using a positive lookbehind, you could use 2 capturing groups and use those in the replacement like $1replacement$2
(^---[\s\S]+?date: )\d{4}\/\d{2}\/\d{2}([\s\S]+?^---)
Regex demo
Explanation
( Capturing group
^---[\s\S]+?date: Match from the start of the line 3 times a - followed by matching any 0+ times any character non greedy and then date:
) Close first capturing group
\d{4}\/\d{2}\/\d{2} Match a date like pattern (Note that this does not validate a date itself)
( Capturing group
[\s\S]+?^--- Match any 0+ times any character non greedy followed by asserting the start of the line and match 3 times -
) Close capturing group
const regex = /(^---[\s\S]+?date: )\d{4}\/\d{2}\/\d{2}([\s\S]+?^---)/gm;
const str = `---
title: test
date: 2018/10/17
description: some thing
---`;
const subst = `$1replacement$2`;
const result = str.replace(regex, subst);
console.log(result);

Answer 3

I'm not sure Javascript supports look behind at all, but if your environment supports it, you can try this regex:
/(?<=---[\s\S]+)(?<=date: )[\d/]+(?=[\s\S]+---)/
It looks behind for '---' followed by anything, then it looks behind for 'date: ' before it matches digits or slash one or more times, followed by a look ahead for anything followed by '---'.
Now you can easily replace the match with a new date.